title: Preliminary crystal structure analysis of a microbial, guanine-specific ribonuclease St at 2.5 A resolution.
abstract: The three-dimensional structure of Ribonuclease St (RNase St), the extracellular ribonuclease from Streptomyces erythreus, has been deduced based on a preliminary electron density map at 2.5 A resolution. RNase St has a substrate specificity similar to ribonuclease T1 which catalyzes the splitting of the phosphodiester bond of guanylic acid. Crystals grown as diamond plates have space group C2 with unit cell parameters a=88.4, b=33.0, c=69.0 A, beta = 98.4 degrees having two enzyme molecules per asymmetric unit. Phases were obtained by use of KAu(CN)4, phenylmercuric acetate and UO2 (CH3COO)2. The overall dimensions of the molecule are 40 X 30 X 25 A. The most prominent secondary structural features are two turns of alpha-helix and a three strand stretch of antiparallel beta-sheet. The alpha-carbon backbone of RNase St seems to have no apparent correlation with that of ribonuclease A.
mesh_terms: D008958:Models, Molecular; D011487:Protein Conformation; D006163:Ribonuclease T1 / Q000302:isolation & purification; D012260:Ribonucleases / Q000302:isolation & purification; D013302:Streptomyces / Q000201:enzymology*; D014961:X-Ray Diffraction
chemical_list: D012260:Ribonucleases; D006163:Ribonuclease T1